1. Field of the Invention
This invention is directed to a coating composition, in particular, to a two component coating composition particularly useful for automotive refinish applications that has an improved pot life but cures rapidly after application to a substrate.
2. Description of the Prior Art
To meet the many requirements of an exterior finish for automobiles and trucks, the automotive industry currently is using multi-layer finishes on automobile and truck bodies and parts. Typical of these finishes are layers of the following: (1) an electrocoat layer applied over a substrate, typically, a phosphatized cold rolled steel, (2) a primer layer, (3) a colored layer, typically pigmented, and (4) a clear layer. A colored top-coat layer may be used in place of the colored layer and clear layer. The repair or refinishing of such multi-layer finishes of automobiles and trucks usually takes place in collision repair facility, for example, a typical automotive repair shop. These facilities generally do not have high temperature ovens but rely on ambient temperature curing, curing with heat lamps, or the use of enclosures that can be heated with forced hot air.
The typical repair coatings that are used are two component compositions that are mixed just prior to spray application. The “pot life” of these coatings is of sufficient length that at least the repair or refinish of the vehicle can be completed and preferably more than one vehicle can be refinished. “Pot life” is the length of time a composition remains at a viscosity at which it can be applied by spraying using conventional equipment. The cure time of these repair coatings must be relatively short so the coated automobile, truck or body part can be move from the area where it is being cured and then must harden in a relatively short time so that it can be polished, sanded or additional coatings, stripes or decals can be applied.
Using conventional formulation techniques, the cure time of a two component coating composition can be reduced by the use of catalyst but a catalyst also shortens the pot life of the composition and often adversely affects the appearance of the resulting finish since the coating after application requires time to flow to form a smooth glossy finish. To improve flow and appearance of the resulting finish, solvents or diluents can be added but these components increase the VOC (volatile organic content) of the coating composition, which according to current air pollution regulations must be kept at a VOC level, which is set forth in local regulation, usually in the range of 2–4 #/gal.
Useful two stage cure coating compositions are disclosed in Lewin et al. U.S. Pat. No. 6,326,059 and Lewin et al. U.S. Pat. No. 6,471,185. Both of these patents require the use of a secondary amine, in particular, tertiary butylamino ethyl methacrylate, for curing. Such amines often cause yellowing of a coating on aging and are very reactive which reduces the pot life of the coating composition and therefore, only restricted amounts of these amines can be used in the coating composition.
It would be desirable to be able to formulate a two component coating composition that has short cure time so that a finish of the coating composition can be sanded or buffed in a relatively short period after application thereby increasing productivity of an automotive or truck refinish facility and still have an excellent appearance and such a coating composition also should have and extended pot life.